I know this is a long time later, but I think I know where @Jeff_Gaudette ran into the issue. As it turns out, you can put a merge code into a URL, but it won’t work if you click Test, it will only work if you publish the campaign and then send out the email normally.
I followed all of @Cheryl_Hunt’s advice and was about to give up when I saw a mention of this on some other website where it seemed they were also embedding merge codes into URLs.
For what it’s worth, you can actually skip the part where you convert the contact field into a campaign field.
So for example, I created a custom field called “QuoteId” and set it to be a whole number. In the email, I checked the merge field for that and used it in a link, so put a URL something like this:
When I sent this using the Test function on the email designer, failure. When I published the campaign anyway and sent it normally (I had it trigger on adding a Tag), worked great.
I’m sure Jeff’s long since past this, but for anyone else that stumbles on this thread, there you go.